[Etherpad] Fix minify version

Minify v6 requires node>=12

https://team.picasoft.net/picasoft/pl/gd5e6y48p3gfupp9tgnchqgw6e

pica-etherpad/Dockerfile

@@ -36,7 +36,7 @@ RUN curl -SL https://github.com/ether/etherpad-lite/archive/${ETHERPAD_VERSION_B
 COPY ./landing-page ./src/templates/landing-page
 
 # Build and replace landing page
-RUN npm install minify && \
+RUN npm install minify@5 && \
     ./src/templates/landing-page/build/build.sh && \
     cp ./src/templates/landing-page/index.html src/templates/index.html && \
     cp -r ./src/templates/landing-page/static/* src/static/